Well I opened up that radio where I was finding 120 AC between the chassis and ground and found out why. Someone put a .25 600V Cap to ground on one side of the AC when it first comes into the radio. Now i still measure a little volage AC and DC when on and off but getting no sparc when I put the ground clip on like I had with that cap in... Why did they put in this cap? From what I hear it was for shunting hi frequency noise to ground. I've seen it on other Radios one was a Rogers.

Hi Bubba,
You are correct about the purpose of this cap. It is the line bypass cap, intended to prevent unwanted RF interference from the AC line. Yours could be original but probably a later replacement. They used a 600v DC rating in order to get adequate AC rating, which is usually around 200v AC on a good 630v film capacitor.

Many radios will work fine without this cap. For replacement the modern trend is to use an "AC safety capacitor" designed for Line-to-ground or Across-the-line duty. If a regular film cap is used it should have a 630v DC rating. In either case I never use one larger than .01 mfd, regardless of what mfd value was used originally.

For a good discussion of this issue see the following webpage:
www.justradios.com/safetytips.html

Poston

:Well I opened up that radio where I was finding 120 AC between the chassis and ground and found out why. Someone put a .25 600V Cap to ground on one side of the AC when it first comes into the radio. Now i still measure a little volage AC and DC when on and off but getting no sparc when I put the ground clip on like I had with that cap in... Why did they put in this cap? From what I hear it was for shunting hi frequency noise to ground. I've seen it on other Radios one was a Rogers.

I think .25uF is much too large for a line bypass. I believe that .005uF would be a more typical value.

I can't imagine that such a large cap would have been called for in the original design. Perhaps somebody in the past was trying to cut interference -- and decided if a little cap does a little good, a big cap should do a LOT of good.

In olden days, they would often use a very high voltage rating for this cap. (I'm presently looking at a Zenith pre-war schematic which calls for a 1000-V, .005uF cap.) The idea was to provide an extra margin of safety against leakage or shorts. The modern practice would be to use an X-Y safety cap for line bypass.

No, everybody, that isn't what that cap is for. A cap ACROSS the line will shunt out unwanted high frequency noise. Choosing various values will eliminate certain interferences, and so you should experiment with values of .05 MFD and lower (sometimes .001 MFD and lower works well for me, depending on the situation).

A condenser from the line cord to the chassis of an AC radio serves to couple the chassis, which is the RF ground, to ground through the AC line. The transformer often provides enough capacitive coupling that this is not needed, but you will sometimes find that reception will improve slightly with this connection. To avoid having to use this condenser, you can ground the radio. The condenser provides a convenient connection without the user having to have excess wires out the back of the set. Unfortunately this condenser also connects the radio to a large source of interference.

Thomas

Hi Thomas,
I must respectfully disagree with your statement "that isn't what that cap is for". Rather than use my own words, I quote the following from page 52 of "Elements of Radio Servicing, Second Edition" by William Marcus & Alex Levy (McGraw-Hill, 1955):

"Condenser C-17 [shown in the accompanying illustration as connected from one leg of the incoming AC line to chassis ground] is the line filter. Its action is to remove various RF line disturbances, such as those caused by sparking brushes on electric motors, from entering the radio. The value of C-17 is not critical. Values ranging from 0.002 to 0.5 mfd are found in various radios."

However, the statement you make about coupling the chassis to ground is true. On page 66 of the above-referenced book, Marcus & Levy make the following additional comments:

"Line filter condenser C-17 is a paper tubular condenser, whose usual capacity is 0.1 mfd. . . The condenser may open, and this would theoretically cause greater interference from line disturbances. An open line filter condenser, however, may cause entirely different effects. Owing to its position in the circuit, the receiver chassis is grounded through condenser C-17 by the lighting mains, one side of which is grounded. The receiver installation may have no ground at all or an indifferent ground, in which case C-17 takes on a new function--that of grounding the receiver. This explains why reception (absence of hum or noise) is often improved by reversing the plug on AC receiver installations. It also explains why a tiny spark or small shock is experienced when connecting a ground to a receiver. When C-17 is open, its grounding function is gone. The most annoying manifestation of this is known as "modulation hum"; that is, the receiver does not hum when making a hum check. The hum comes on as a station is tuned in. There will be no hum between stations. Standard procedure for modulation hum is to check the ground and condenser C-17. Bridging condenser C-17 will another condenser of like value is a check for an open condenser."

Poston

:No, everybody, that isn't what that cap is for. A cap ACROSS the line will shunt out unwanted high frequency noise. Choosing various values will eliminate certain interferences, and so you should experiment with values of .05 MFD and lower (sometimes .001 MFD and lower works well for me, depending on the situation).
:
:A condenser from the line cord to the chassis of an AC radio serves to couple the chassis, which is the RF ground, to ground through the AC line. The transformer often provides enough capacitive coupling that this is not needed, but you will sometimes find that reception will improve slightly with this connection. To avoid having to use this condenser, you can ground the radio. The condenser provides a convenient connection without the user having to have excess wires out the back of the set. Unfortunately this condenser also connects the radio to a large source of interference.
:
:Thomas

Yes I was told along time ago that it was for shunting stray Frequencies to ground. It isn't on the schematic of the 88K but it is the same brand and vintage Aerovox paper cap that the others in the radio are,, so not sure if it's stock. ALL I KNOW IS I am getting no sparks when I put the ground clip on now and before I was getting quite a nasty spark. But only with the plug one way I guess when the hot was on the same side as the cap to ground...

:Hi Thomas,
:I must respectfully disagree with your statement "that isn't what that cap is for". Rather than use my own words, I quote the following from page 52 of "Elements of Radio Servicing, Second Edition" by William Marcus & Alex Levy (McGraw-Hill, 1955):
:
:"Condenser C-17 [shown in the accompanying illustration as connected from one leg of the incoming AC line to chassis ground] is the line filter. Its action is to remove various RF line disturbances, such as those caused by sparking brushes on electric motors, from entering the radio. The value of C-17 is not critical. Values ranging from 0.002 to 0.5 mfd are found in various radios."
:
:However, the statement you make about coupling the chassis to ground is true. On page 66 of the above-referenced book, Marcus & Levy make the following additional comments:
:
:"Line filter condenser C-17 is a paper tubular condenser, whose usual capacity is 0.1 mfd. . . The condenser may open, and this would theoretically cause greater interference from line disturbances. An open line filter condenser, however, may cause entirely different effects. Owing to its position in the circuit, the receiver chassis is grounded through condenser C-17 by the lighting mains, one side of which is grounded. The receiver installation may have no ground at all or an indifferent ground, in which case C-17 takes on a new function--that of grounding the receiver. This explains why reception (absence of hum or noise) is often improved by reversing the plug on AC receiver installations. It also explains why a tiny spark or small shock is experienced when connecting a ground to a receiver. When C-17 is open, its grounding function is gone. The most annoying manifestation of this is known as "modulation hum"; that is, the receiver does not hum when making a hum check. The hum comes on as a station is tuned in. There will be no hum between stations. Standard procedure for modulation hum is to check the ground and condenser C-17. Bridging condenser C-17 will another condenser of like value is a check for an open condenser."
:
:Poston
:
:
:
::No, everybody, that isn't what that cap is for. A cap ACROSS the line will shunt out unwanted high frequency noise. Choosing various values will eliminate certain interferences, and so you should experiment with values of .05 MFD and lower (sometimes .001 MFD and lower works well for me, depending on the situation).
::
::A condenser from the line cord to the chassis of an AC radio serves to couple the chassis, which is the RF ground, to ground through the AC line. The transformer often provides enough capacitive coupling that this is not needed, but you will sometimes find that reception will improve slightly with this connection. To avoid having to use this condenser, you can ground the radio. The condenser provides a convenient connection without the user having to have excess wires out the back of the set. Unfortunately this condenser also connects the radio to a large source of interference.
::
::Thomas

Everybody,
Thanks for a quick education. I was never good at power engineering, but Posten's suggestion of a site was most educational, and made me rethink my simple solution to the same problem in troubleshooting my Edison 7R/8P. In my ignorance, I was switching the common side of the line and had a cap to chassis on the hot side. Turn on the set, hook up the test equip and no problem. Turn off the set with test equip still connected, and the GFI trips. A cap on the low side of the line to chassis helps a bit, but this modulation hum is a new area for me. I fought all kinds of interference, but it was 90% radiated vs line conducted.
marv

:Yes I was told along time ago that it was for shunting stray Frequencies to ground. It isn't on the schematic of the 88K but it is the same brand and vintage Aerovox paper cap that the others in the radio are,, so not sure if it's stock. ALL I KNOW IS I am getting no sparks when I put the ground clip on now and before I was getting quite a nasty spark. But only with the plug one way I guess when the hot was on the same side as the cap to ground...
:
:
:
:
::Hi Thomas,
::I must respectfully disagree with your statement "that isn't what that cap is for". Rather than use my own words, I quote the following from page 52 of "Elements of Radio Servicing, Second Edition" by William Marcus & Alex Levy (McGraw-Hill, 1955):
::
::"Condenser C-17 [shown in the accompanying illustration as connected from one leg of the incoming AC line to chassis ground] is the line filter. Its action is to remove various RF line disturbances, such as those caused by sparking brushes on electric motors, from entering the radio. The value of C-17 is not critical. Values ranging from 0.002 to 0.5 mfd are found in various radios."
::
::However, the statement you make about coupling the chassis to ground is true. On page 66 of the above-referenced book, Marcus & Levy make the following additional comments:
::
::"Line filter condenser C-17 is a paper tubular condenser, whose usual capacity is 0.1 mfd. . . The condenser may open, and this would theoretically cause greater interference from line disturbances. An open line filter condenser, however, may cause entirely different effects. Owing to its position in the circuit, the receiver chassis is grounded through condenser C-17 by the lighting mains, one side of which is grounded. The receiver installation may have no ground at all or an indifferent ground, in which case C-17 takes on a new function--that of grounding the receiver. This explains why reception (absence of hum or noise) is often improved by reversing the plug on AC receiver installations. It also explains why a tiny spark or small shock is experienced when connecting a ground to a receiver. When C-17 is open, its grounding function is gone. The most annoying manifestation of this is known as "modulation hum"; that is, the receiver does not hum when making a hum check. The hum comes on as a station is tuned in. There will be no hum between stations. Standard procedure for modulation hum is to check the ground and condenser C-17. Bridging condenser C-17 will another condenser of like value is a check for an open condenser."
::
::Poston
::
::
::
:::No, everybody, that isn't what that cap is for. A cap ACROSS the line will shunt out unwanted high frequency noise. Choosing various values will eliminate certain interferences, and so you should experiment with values of .05 MFD and lower (sometimes .001 MFD and lower works well for me, depending on the situation).
:::
:::A condenser from the line cord to the chassis of an AC radio serves to couple the chassis, which is the RF ground, to ground through the AC line. The transformer often provides enough capacitive coupling that this is not needed, but you will sometimes find that reception will improve slightly with this connection. To avoid having to use this condenser, you can ground the radio. The condenser provides a convenient connection without the user having to have excess wires out the back of the set. Unfortunately this condenser also connects the radio to a large source of interference.
:::
:::Thomas

Well, as I said before, the cap ACROSS the line is NOT critical in value, but from MY EXPERIENCE, different values have different effects, whether they be elimination of different types of static, or elimination of static on different parts of the dial. As static is actually a radio frequency range, however broad, different values of condensers, especially those smaller condensers that would be close to these frequencies, will have different effects on the static. Since I do believe that I live in the here and now, and not in a hallucinogenic world, I know that what I experienced must be true. However, as you say, the value is not critical, as I also said.

As for the condenser from the line to the chassis (not across the line), unless you also ground the chassis and happen to have the plug inserted so that the condenser is on the hot side, this condenser will not shunt interference to ground. If the radio is not grounded, it will not shunt interference anywhere. All it will do is capacitively connect the chassis to the line. Depending on how you plug the radio in, you may or may not feed a bunch of interference into the radio from the line (usually from the hot side). I have never read that this condenser was to omit interference, and cannot immagine how it would do so. The only way to remove interference is to either filter it out with a choke or to bypass it around the radio by placing a condenser ACROSS the line cord so that the interference goes from the hot to the neutral (ground), and not into the radio. From my personal experience, all that I have just stated is all that I have ever seen or heard. If I removed such a condenser (one that goes from the line to the chassis), any interference had was greatly reduced, as there was now no connection other than through the power transformer that would allow interference to get into the radio from the line. The adverse effect of this, of course, was also a reduced sensitivity of the radio, as there was now no strong connection to ground other than through the capacitive coupling through the power transformer. A connection to a water pipe or other ground not at all associated with the AC line greatly remedied this. Also I have found that placing a filter choke on both sides of the line reduced interference, but also reduced reception on most of my AC radios (and AC-DC radios), due to the filtering out of all radio frequencies by the chokes. By doing this, there was no available ground for the radio. The only way to correct this was to either only choke one side of the line and see to it that this side was always connected to the hot, or to ground the radio by some other means. Choking one side of the line, of course, still allows any interference in the cold side to enter the radio. This interference is usually not as high as that found in the hot side of the line.

It is perfectly normal for a radio with a condenser from the line to the chassis to spark lightly when the chassis is grounded. As a condenser passes AC, it will pass AC to the chassis if the radio is plugged in with the condenser on the hot side. The chassis may also give you a slight shock due to this. It is also normal for the spark to be rather large if a .1 MFD condenser is used. As the condenser size is increased, it will pass more and more low frequencies. Since 60 cycles AC is a low frequency, this is what you are approaching as you increase the value of the condenser. A smaller condenser, say .05, would produce less of a spark. These sparks are not normally harmful to equipment. If you were to approach say 10 MFD, you may start a heavy current draw that would be dangerous.

I hate to look like the fool on the hill, as the Beatles put it, but I have read quite a few articles on radio performance that were greatly misinforming. I have read some others that were incredibly good and accurate (thorough, thoughtful, and otherwise astonishing). I must disagree with the idea that a condenser from the line to the chassis (not across the line) is actually to reduce radio interference. I strongly encourage all who disagree with me to reread what I have said thoroughly and to try what I have said for yourselves to see exactly what I mean.

Thomas

:Well, as I said before, the cap ACROSS the line is NOT critical in value, but from MY EXPERIENCE, different values have different effects, whether they be elimination of different types of static, or elimination of static on different parts of the dial. As static is actually a radio frequency range, however broad, different values of condensers, especially those smaller condensers that would be close to these frequencies, will have different effects on the static. Since I do believe that I live in the here and now, and not in a hallucinogenic world, I know that what I experienced must be true. However, as you say, the value is not critical, as I also said.
:
:As for the condenser from the line to the chassis (not across the line), unless you also ground the chassis and happen to have the plug inserted so that the condenser is on the hot side, this condenser will not shunt interference to ground. If the radio is not grounded, it will not shunt interference anywhere. All it will do is capacitively connect the chassis to the line. Depending on how you plug the radio in, you may or may not feed a bunch of interference into the radio from the line (usually from the hot side). I have never read that this condenser was to omit interference, and cannot immagine how it would do so. The only way to remove interference is to either filter it out with a choke or to bypass it around the radio by placing a condenser ACROSS the line cord so that the interference goes from the hot to the neutral (ground), and not into the radio. From my personal experience, all that I have just stated is all that I have ever seen or heard. If I removed such a condenser (one that goes from the line to the chassis), any interference had was greatly reduced, as there was now no connection other than through the power transformer that would allow interference to get into the radio from the line. The adverse effect of this, of course, was also a reduced sensitivity of the radio, as there was now no strong connection to ground other than through the capacitive coupling through the power transformer. A connection to a water pipe or other ground not at all associated with the AC line greatly remedied this. Also I have found that placing a filter choke on both sides of the line reduced interference, but also reduced reception on most of my AC radios (and AC-DC radios), due to the filtering out of all radio frequencies by the chokes. By doing this, there was no available ground for the radio. The only way to correct this was to either only choke one side of the line and see to it that this side was always connected to the hot, or to ground the radio by some other means. Choking one side of the line, of course, still allows any interference in the cold side to enter the radio. This interference is usually not as high as that found in the hot side of the line.
:
:It is perfectly normal for a radio with a condenser from the line to the chassis to spark lightly when the chassis is grounded. As a condenser passes AC, it will pass AC to the chassis if the radio is plugged in with the condenser on the hot side. The chassis may also give you a slight shock due to this. It is also normal for the spark to be rather large if a .1 MFD condenser is used. As the condenser size is increased, it will pass more and more low frequencies. Since 60 cycles AC is a low frequency, this is what you are approaching as you increase the value of the condenser. A smaller condenser, say .05, would produce less of a spark. These sparks are not normally harmful to equipment. If you were to approach say 10 MFD, you may start a heavy current draw that would be dangerous.
:
:I hate to look like the fool on the hill, as the Beatles put it, but I have read quite a few articles on radio performance that were greatly misinforming.
I removed the cap..

I have read some others that were incredibly good and accurate (thorough, thoughtful, and otherwise astonishing). I must disagree with the idea that a condenser from the line to the chassis (not across the line) is actually to reduce radio interference. I strongly encourage all who disagree with me to reread what I have said thoroughly and to try what I have said for yourselves to see exactly what I mean.
:
:Thomas